Code question?

[SJones]

Can someone tell me what the the exclamation is doing for the (isset) function - example /

<?php
if (!isset($_GET['p'])) { // no page specified -> load content from default.txt

• I am just learning but in this book all lf the (isset) function never has the "!" in front of it. Please explain???

[rehfeld]

it negates the condition

if (isset($var)) {

}

"if $var IS set"

if (!isset($var)) {

}

"if $var is NOT set"

[SJones]

Thank you!
I appreciate your answering